IT WAS last spring that White House drug czar Barry McCaffrey started a campaign warning people that some of the body-building supplement androstenedione on the market was adultered and contained testosterone.

The data was forwarded to the Drug Enforcement Administration, which has started an investigation into the matter, persons familiar with the informal probe told The Post yesterday.

Testosterone, considered a steroid, is available only by prescription and its presence in over-the-counter andro is illegal.

At the time, many wondered what effect, if any, news of adultered andro being sold, much of it over the Internet to anyone with access to a credit card, would have on the most famous user of the product, Mark McGwire.

Well, we now know. Big Red quit andro.

While it may or may not have anything to do with the drug czar’s campaign, McGwire did reveal before Wednesday night’s game that he has been off andro for four months – or roughly since McCaffrey started making waves.

Andro is metabolized by the body into testosterone. It is banned by the Olympics, the NCAA, the NFL and NBA, while allowed in baseball.

When it was learned last year that McGwire used andro and creatine to build his biceps and Popeye forearms, sales of the two supplements soared. Andro sales quadrupled to an estimated $40 million.

While that is still a small part of the $1.2 billion sports nutrition business, that young men and teens were lining up to purchase the product so they could be like Mark unsettled McGwire.

“I thought long and hard about it and I don’t like the way it was portrayed like I was the endorser of the product, which I wasn’t,” McGwire told reporters. “I don’t like how it’s portrayed, but young kids take it because of me. I don’t like that.”

McGwire said he would “discourage young people from taking it.”

Playing without andro doesn’t seem to have hurt McGwire’s strength. Over the last 21 games, he has hit 22 homers.

McCaffrey applauded McGwire’s action and said the slugger’s action sends a “powerful” message to young people.

McCaffrey and the DEA are still testing andro to see if it meets science’s three-prong test for it being a steroid itself. Baseball is testing it, too. Those results are due out this fall.

Greg Sera, a sales rep at the Great Earth Vitamin store in the Roosevelt Mall, like most retailers, didn’t recommend andro to teens and young men.

“It’s for hard-core middle-aged body-builders,” he said.

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A small space on Met co-owner Fred Wilpon’s full plate may about to be cleared.

A deal to purchase the St. Catherines Stompers A-League team and bring it to Brooklyn is expected to be announced in the next 10 days to two weeks, The Post has learned. An agreement in principle is already signed, and while things can always go awry, both sides seem intent on getting this deal out of the way.

If so, Wilpon will only have to deal with his talks with City Hall to build a new stadium in Queens, negotiations with Cablevision concerning the sale of the team and all those phone calls from long-lost friends looking for tickets to see his suddenly-first-place Mets.